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June 1935 Radio-Craft
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
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The year 1935 could be considered
the beginning of a new paradigm in communications thanks to the introduction of metal-encased
vacuum tubes. They facilitated a move into higher frequency circuit design and denser
component placement (smaller volume). Prior to then, vacuum tubes were almost exclusively
encased in a glass envelope with no innate guard against the emission or absorption
of electromagnetic fields from nearby components. Metal-encased tubes provide benefits
like better heat dissipation, smaller physical size, ruggedness, inherent RF shielding,
and lower parasitic values of capacitance and inductance due to smaller plate areas
and shorter lead lengths, respectively. The highest barrier to widespread adoption
of metal tubes, history would show, was the higher cost of production that made
consumer products more expensive at a time when not every household saw the need
for a radio or, eventually, a television.
Now - Metal Tubes
 As first announced by Radio-Craft, last
month, the new all-metal tubes are making their appearance. Many advantages are
claimed for the new tubes, as explained.
Since the brief mention made on page 646 of May 1935 Radio-Craft of the new metal
tubes, definite information has been released by the General Electric Co. who developed
the new method of manufacture for the RCA Manufacturing Co., Inc.
There are many advantages claimed for the new tubes. They are smaller in size,
being 1 in. in diameter at the largest point and varying between 5/8-in, to 3 inches
high (above the base or sub-panel). They provide their own shielding by virtue of
the metal shell and this shell is a better heat conductor and radiator than glass.
Because of the method of construction, the leads are shorter, within the tube, which
permits greater amplification at the higher frequencies (the inter-electrode capacity
is approximately 1/3 of the value for the equivalent glass tube capacity) and the
more effective shielding assures greater stability.
The mechanical construction of the new tubes is evident from
these detailed sketches of a screen-grid type.
Ten types of these metal tubes have been developed, to date, having 6.3 V filaments.
Some of these tubes have identical characteristics with present glass tubes (some
of them are shown in the photo above, along with their glass twins) though several
new types are planned including a double diode and a hexode which is an improved
pentagrid converter.
The new tubes have one more base pin than comparable glass tubes, since the metal
envelope has become a shield, and provision must therefore be made to ground this
envelope. Designers of the tubes have even taken into consideration ease of inserting
them in their sockets. The contact pins are all the same diameter and in the center
is a longer, larger, insulated keyed pin. By placing this insulated pin in a hole
centrally located in the socket and rotating the tube until the key slips into its
groove, the tube is quickly and easily inserted. (See "Radio Trends," by Hugo Gernsback.
page 69, August 1934 Radio-Craft!)
The 8-pin base and socket have been standardized for all the metal tubes; when
less than 8 pins are needed, they are simply omitted from the tube base and, of
course, the corresponding contact on the socket is not wired into the circuit. This
is a revolutionary change in tube manufacture.
In appearance the metal tubes are cylindrical in form, some having a reduced
diameter at the top. Others, such as a radio-frequency amplifier, have a terminal
at the top extremity. Each lead-in wire passes through a tiny bead of special glass
that is fused securely within an alloy eyelet, which in turn is welded to the metal
container, thus assuring a long life vacuum. This alloy, having substantially the
same coefficient of expansion as glass, is known as "Fernico" and is a combination
of iron (fer) nickel (ni) and cobalt (co). It was developed expressly for this purpose
of a perfect seal in the new tubes.
The inner parts of the tube are first assembled on the steel end plate or "header;"
The shell is placed over the assembly and welded to the header at its circumference.
This welding is done with Thyratron controlled welders and takes about 1/20-second
to complete: about 20,000 to 30,000 amperes flow through the metal when the weld
is being made.
During evacuation, the occluded gases in the tubes are removed by heating the
entire tube in a gas burner to a red heat. Design engineers claim that the metal
envelope helps to "clean up" these molecules of gas, acting as a sort of sponge,
thus producing a more complete vacuum.
Elimination of the glass "pinch seal" in which all leads and supports are concentrated
in the glass tubes, allows the leads to enter the header of the new tube at the
proper points for short direct paths. The metal shields are, of course, much stronger
than glass bulbs, and not subject to breakage, while the use of short stiff supports
results in less mechanical vibration of the elements.
The familiar metal shield which is necessary with the glass tube in radio frequency
portions of a circuit is no longer required with the new tube. The metal envelope
itself serves as a shield. And since closer proximity of shield to elements is realized,
the shielding is more effective.
No details are available yet, about the price of the new tubes compared to equivalent
glass types. However, the methods of making them, and the simplified machines required
indicate that the price will be somewhat lower than glass tubes when production
reaches its full height. Manufacture of the new tubes will start early in the summer
- but it is expected that the first sets to use them will be the fall models of
the G.E. line.
In a preliminary folder issued by RCA Manufacturing Co. Inc., six of the new
metal tubes are identified with type numbers. The 6A8 is listed as a pentagrid converter
(similar to 6A7) - the 6C5 is a triode (similar to the 76) - the 6D5 is a power
amplifier triode (equivalent to the 45 but having a 6.3 V., 0.7 A. filament) - 6H6
is a twin diode (this tube has no equivalent in glass tubes) - 6J7 is a triple-grid
detector-amplifier (somewhat similar to the 6C6 - though the characteristics are
not identical) and the 6K7 is an R.F. pentode with remote cut-off (similar to the
6D6 glass tube).
Posted July 20, 2023
(updated from original post
on 12/31/2015)
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